1. Field of the Invention
This invention relates to a process of producing quicklime by a calcining treatment with hot gases in a rotary kiln, wherein solid carbonaceous fuel for producing hot gases is fed into the charging end of the rotary kiln and oxygen-containing gases are blown through nozzle blocks into the charge disposed over nozzle blocks in the heating-up zone beginning at the point where ignitable particles of the solid fuel first appear.
2. Discussion of Prior Art
Quicklime having a high reactivity is increasingly made in a rotary kiln although the latter has a higher heat consumption than the shaft furnace. But the rotary kiln produces lime of higher and more uniform quality and can process particles smaller than 30 mm. Such lime, which is particularly used to make electric steel, should also have a low sulfur content. This sulfur content is mainly due to the sulfur content of the fuels which have been used. The sulfur content of the quicklime can be minimized primarily by the use of low-sulfur fuel and also by heating the limestone in the rotary kiln with hot combustion gases produced in a central burner or in shell burners or shell nozzles, i.e., so as to avoid a direct contact between the unburnt fuel and the charge (Zement-Kalp-Gips, No. 2, 1969, pages 75 to 81; French Pat. No. 1,487,240; U.S. Pat. No. 2,941,791; German Pat. No. 618,872.
Because low-sulfur fuels are more expensive than high-sulfur fuels, the use of the former adds to the costs of the process particularly because it has a high heat requirement. In a rotary kiln there is only a very low heat transfer from the flames and hot gases of the kiln atmosphere to the solid charge, particularly in the heating-up zone, because heat is mainly transferred on the surface of the charge. As a result, the exhaust gases have a high content of heat which has been produced by a combustion of relatively expensive fuels. This is also true for the processes in which the sulfur content of the quicklime is minimized by calcining in a neutral or reducing atmosphere (German Pat. No. 1,108,603; Czechoslovakian Pat. No. 127,978). That process also involves a risk of temperature fluctuations and hot spots if the oxygen supply varies. That risk can be avoided only in part by the provision of expensive sealing means preventing an infiltration of air.
It is known from German Offenlegungsschrift No. 2,719,347 to reduce the heat requirement of a rotary kiln for producing a highly reactive, low-sulfur lime in that solid carbonaceous material is fed through the charging end of the rotary kiln and oxygen-containing gases are blown through nozzle blocks into the charge disposed over nozzle blocks and through shell tubes into the free kiln space in that region of the rotary kiln which begins with the appearance of ignitable particles of the fuel and ends up to 50% of the kiln length. The succeeding region of the kiln is supplied with oxygen-containing gases through shell tubes or with additional fuel through shell burners or a central burner. In that process the heating-up zone of the rotary kiln is much reduced in length and the total energy consumption is much reduced also because the heat content of the volatile combustible constituents of the solid fuel is utilized to a large extent. As a result, more high-sulfur fuel can be used to produce a lime having a given sulfur content or the use of a solid fuel having a given sulfur content will result in a lower sulfur content of the lime because less solid fuel is employed.
It is an object of the invention further to improve the process described hereinbefore and further to decrease the heat requirement of a rotary kiln used to produce highly reactive, low-sulfur quicklime.